This invention is in the field of illumination and more specifically relates to illumination of indicators, such as indicators with dial pointers.
Illumination of display indicators in vehicles is accomplished by either externally directing flood lighting to the indicators or by integral lighting of the indicators. Integral lighting usually involves a source of illumination, such as a light bulb or light directed to the indicator by fiber optics (also using a light bulb source). It is also possible to use back lighting, in which light eminates from behind the surface of the indicator. In that case, separate lighting is required for moveable members, such as dial pointers. The increased popularity of "night vision goggles" and other passive night vision aids, creates unique problems with illumination of cockpit indicators. These passive night vision aids operate by amplifying available light, particularly in the longer wave lengths of visible light. In the upper infrared ranges illumination of cockpit indicators from ordinary incandescent sources overwhelms the sensor elements which are used in such night vision aids, and thereby interrupts the night vision aid for several minutes. It is important that the display indicators remain illuminated, not only for the benefit of the crewmen who are not wearing night vision aids, but also because those using the goggles will typically view the instruments by looking under the goggles. In an attempt to alleviate this problem, attempts have been made to filter the incandescent light. Filtration of the objectionable light must be very efficient because small amounts of light within the active frequency range of the night vision aid will overwhelm the aid, due to the close proximity to the night vision aid of the illuminated indicators.
Conventional passive night vision aids are sensitive to light having wave lengths of 580 nM or longer. This means that illumination of displays at shorter wave lengths (frequencies above the frequency range of the passive night vision aids) will not substantially interfere with the night vision aids and will facilitate the use of such night vision aids.
As previously indicated, filtration of incandescent light is often inadequate. Where each indicator has its own individual light sources, such filtration can become very cumbersome. Electroluminescent lighting is therefore a preferred method of illuminating such displays. Efficient electroluminescent displays are available which transmit most of the optical energy at shorter wave lengths than the 580 nM frequency. A reasonable amount of filtration can be used to filter any light transmitted at wave lengths above the 580 nM cut-off point.
One problem with electroluminescent lighting of displays is that the background of the display can be easily illuminated, but illumination of moveable elements, such as pointers, is often difficult. Thus, electroluminescent lighting is ideal for back lighting of liquid crystal displays and for controls, where the back-lit area is not moved and is not sensitive to frictional forces. In the case of controls, the indicator, being manually operated, can carry an electroluminescent light source, connected by wires or slip rings. On the other hand, dial pointers are often connected to sensitive movements. The use of slip rings and connecting wires can have a detrimental effect on the performance and reliability of such indicators. One method of illuminating such a display is to illuminate the display from the perimeter of the display, with the dial pointer being configured to reflect the light from the side of the display. The effect of such an illumination technique is to provide a substantial amount of lighting for the background of the display (the numera1s and graduations) while providing very little illumination for the pointer.
For these reasons, it is desired to provide a more convenient and efficient technique for illuminating moveable pointer indicators at light frequencies which do not extend past (below) a predetermined limit. It is a further object of the invention to provide a method for directly illuminating the moveable pointer to a display with illumination that fits within the above frequency criteria. It is a further object of the invention to provide a method for using electroluminescent light sources for illuminating a moveable pointer on a display, without interfering with the movement of the moveable pointer.